Much of our present understanding of the functional organization of the sensory visual system has been gained from an understanding of the detailed anatomical connections between neurones at successive levels of the system and comparisons with the physiological properties of individual neurones at each level. Far less is known, however, about the organization of the motor visual system or about the pathways and mechanisms by which visal information, as well as information derived from other sensory systems, influences oculomotor activity. My long-term goal is to understand the structural and functional organization of the oculomotor system and the ways in which motor units of the extraocular muscles are related to various premotoneuronal centers and their pathways involved in eye movements. I am approaching the problem in the cat, by studying the neuronal and synaptic organization of the abducens nucleus. The localization and identification of retractor bulbi motoneurones will be studied by light and electron microscopy following labelling with horseradish peroxidase. The identification of sources of afferents to and their synaptic terminations on abducens motoneurones and interneurones will be studied by labelling with horseradish peroxidase and electron microscope autoradiography, as a prerequisite to ultimately examining the patterned distribution of specific afferents onto abducens neurones whose electrophysiological properties are known.